The present invention relates to equipment for servicing oil and gas wells and, in particular to an apparatus and method for protecting equipment in a wellhead from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment, while permitting tubing to be run in or out of the well.
Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow in order to make or keep them economically viable. The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand.
The components which make up the wellhead such as the valves, tubing hanger, casing hanger, casing head and the blowout preventer equipment are generally selected according to the characteristics of the particular well and are not capable of withstanding the fluid pressures required for well fracturing and stimulation procedures. Wellhead components are available that are constructed to withstand high pressures, but it is not economical to equip every well with them.
There are many wellhead isolation tools used in the field that conduct corrosive and abrasive high pressure fluids and gases through the wellhead components to prevent damage to the wellhead.
The wellhead isolation tools known in the prior art generally insert a mandrel through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment, in order to stimulate production. A top end of a mandrel is connected to one or more high pressure valves, through which the stimulation fluids are pumped. In some applications, a pack-off assembly is provided at a bottom end of the mandrel for achieving a fluid seal against an inside of the production tubing or casing so that the wellhead is completely isolated from stimulation fluids. One such tool is described in Applicant""s U.S. Pat. No. 4,867,243 which issued Sep. 19, 1989, and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME. The length of the mandrel need not be precise because the location of the pack-off assembly in the production tubing or casing is immaterial, so long as a pack-off assembly is sealed against the inner wall of the production tubing or casing. Consequently, variations in the length of the wellhead of different oil or gas wells are of no consequence.
In an alternate wellhead isolation tool configuration, the mandrel in an operative position requires fixed-point pack-off in the well. The mandrel includes an annular sealing body attached to the bottom end of the mandrel for sealing against a bit guide which is mounted on the top of a casing in the wellhead. A mechanical lock-down mechanism secures the mandrel against the bit guide. The annular sealing mechanism and the mechanical lock-down mechanism are described in Applicant""s U.S. Pat. No. 4,867,243, which issued on Sep. 19, 1989 and is entitled BLOWOUT PREVENTER PROTECTOR AND SETTING TOOL. This tool is inserted into the operative position using, for example, the setting tool described in Applicant""s above-referenced U.S. Pat. No. 4,867,243. Although this setting tool works very well and has been repeatedly proven in high pressure well conditions, the setting tool has to be removed to provide access to the isolation tool. This requires time and equipment and slows down transition to the well stimulation process.
It is therefore desirable to integrate injectors with the tool for inserting the mandrel through the wellhead. An example of an isolation tool with integrated injectors is described in U.S. Pat. No. 4,241,786, which issued to Bullen on Dec. 30, 1980 and is entitled WELL TREE SAVER. Bullen""s tool includes two hydraulic cylinders supported on a hydraulic cylinder mounting plate and off-set from the wellhead. The piston rods of the hydraulic cylinders are connected at their free ends to a base plate mounted to the top of the wellhead. The cylinder mounting plate bears a high pressure tube and a high pressure valve attached to the top of the high pressure tube. The high pressure valve extends upright from the hydraulic cylinder mounting plate and the top ends of the hydraulic cylinders are connected together by a cross member. Consequently, access to the high pressure valve is restricted and it is difficult or impossible to mount other equipment to the top of the high pressure valve, such as a blowout preventer, coil tubing injector, etc.
There is therefore a need for an improved well stimulation tool that includes integral injectors, while providing unobstructed access to a top of the tool.
It is an object of the present invention to provide an apparatus for protecting equipment in a wellhead from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment, and which is incorporated with injectors so that no additional injectors are needed to place the apparatus into an operative position.
It is another object of the present invention to provide an apparatus for protecting equipment in a wellhead from exposure to fluid pressures, abrasives and corrosive fluid used in a well treatment while permitting other equipment required in the well treatment to be mounted thereabove.
In accordance with one aspect of the present invention, an apparatus is provided for protecting equipment in a wellhead from exposure to fluid pressure, abrasives and corrosive fluids used in a well treatment to stimulate production. The apparatus includes a tool adapted to be inserted through a wellhead to an operative position. The tool includes a base plate adapted to be connected to a top end of the wellhead and a top plate at a top end thereof for supporting equipment selectively mounted to the tool. A pair of injectors is provided in a parallel relationship and is located at opposed sides of the respective base and top plates. Each injector has opposed ends secured to the base plate and the top plate, respectively, so that the injectors can move a portion of the tool reciprocally through the wellhead into and out of the operative position when the base plate is connected to the top end of the wellhead. The injectors preferably include a pair of hydraulic injectors. Each hydraulic injector includes a cylinder and a piston rod extendable from the cylinder, thereby forming a cylinder end and a rod end of the hydraulic injector. The cylinder end is secured to and flush with the top plate and the rod end is connected to the base plate.
More especially, according to an embodiment of the present invention, the tool includes a base plate adapted to be inserted into a top of the wellhead and a mandrel adapted to be inserted down through the wellhead to the operative position. The base plate preferably includes a fluid seal through which the mandrel is reciprocally movable in a passage extending therethrough. The mandrel includes an annular sealing body secured to a bottom end thereof for sealing engagement within a tubing head spool above a bit guide at a top of a casing of the well when the mandrel is in the operative position. A mandrel head is preferably connected to a top end of the mandrel, and includes a passage extending therethrough and in fluid communication with the mandrel. The mandrel head is releasably secured for example, by a lock nut around the mandrel, to the base plate when the mandrel is inserted through the wellhead into the operative position. The tool preferably further includes a high pressure valve mounted to the mandrel head and in fluid communication with the passage of the mandrel head. An adapter is preferably mounted to the top of the high pressure valve and has a central passage in fluid communication with the high pressure valve. The adapter is secured to the top plate so as to connect the equipment selectively mounted to the high pressure valve.
In accordance with another embodiment of the present invention, the tool further includes a fracturing head located between the high pressure valve and the mandrel head. The fracturing head includes an axial passage in fluid communication with the mandrel and the high pressure valve, and at least one radial passage in fluid communication with the axial passage.
The annular sealing body preferably includes at least one O-ring attached therearound to seal a gap between the annular sealing body and an interior wall of the tubing head spool. Thus, the bottom end of the annular sealing body is adapted to rest on the top of the bit guide to bear the entire weight of the apparatus, the equipment mounted on the top of the apparatus, and even the weight of a tubing string.
In accordance with another aspect of the invention, a method is described for protecting equipment in a wellhead from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment to stimulate production. The method comprises: a) suspending above the wellhead an apparatus which includes a tool having a mandrel adapted to be inserted through a wellhead to an operative position, the tool including a base plate adapted to be connected to a top end of the wellhead and a top plate at a top end of thereof, and which further includes a pair of injectors in a parallel relationship located at opposed sides of the respective base and top plates, each injector having opposed ends secured to the base plate and the top plate, respectively; b) aligning the mandrel with the wellhead and lowering the apparatus until the apparatus rests on the wellhead, and mounting the base plate to the wellhead; c) opening a fluid flow control mechanism of the wellhead to permit access to a well bore; d) actuating the injectors to insert the mandrel through the wellhead into an operative position in which an annular sealing body at a bottom end of the mandrel is in a fluid sealing engagement within a tubing head spool above a bit guide at a top of a casing of the well; e) locking the mandrel in the operative position; and f) connecting the apparatus to a high pressure fluid line for well stimulation.
In accordance with a further aspect of the present invention, a method is described for running a tubing string into a well while protecting equipment in a wellhead from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment or well service operation. The method comprises: a) mounting to the wellhead a base plate of an apparatus, the apparatus including a tool having a mandrel adapted to be inserted through a wellhead to an operative position, the tool further including a fracturing head having an axial passage and at least one radial passage in fluid communication with the axial passage, a high pressure valve mounted to the fracturing head and a top plate connected to a top of the high pressure valve, and the apparatus further including a pair of injectors in a parallel relationship located at opposed sides of the respective base and top plates, each injector having opposed ends secured to the base plate and the top plate, respectively; b) aligning the mandrel with the wellhead and lowering the apparatus until the apparatus rests on the wellhead, and mounting the base plate to the wellhead; c) opening a fluid flow control mechanism of the wellhead to permit access to a well bore; d) actuating the injectors to insert the mandrel through the wellhead into an operative position in which an annular sealing body at a bottom end of the mandrel is in a fluid sealing engagement within a tubing head spool above a bit guide at a top of a casing of the well; e) locking the mandrel in the operative position; and f) running the tubing string into the wellbore through at least one blowout preventer mounted to the top plate and in fluid communication with the high pressure valve of the apparatus.
The apparatus of the present invention has a relatively simple configuration and provides direct access to the well so that the use of the apparatus is extended to a wide range of well service applications. The apparatus of the present invention advantageously permits the tubing string to run in or out of the well without moving the apparatus from the wellhead. The tubing string can even be moved up or down in the well while well treatment fluids are being pumped into the well. Labour and the associated costs are thus reduced.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter.